feat: screen sharing via pixelpass (Discord-style, presence-borne ticket)

Surface pixelpass screen-sharing from inside a peerspeak room. peerspeak owns
voice, pixelpass owns pixels — they're never Cargo deps of each other; the
contract is pixelpass's CLI flags + its `--output json` stdout stream.

Modelled on Discord: multiple simultaneous sharers, a 🔴 Live badge + 👁 Watch
on each sharing peer's card, and in-progress shares visible to late joiners.

- New `src/screenshare` module: pure `parse_pixelpass_event` seam + `pixelpass_path`
  discovery (13 unit tests), async `spawn_host` (→ ticket) and `spawn_viewer`
  (→ parse connected{url} → open mpv, vlc fallback). No new deps.
- Sharing rides presence: `PeerState.sharing: Option<ticket>` (serde-defaulted),
  so the existing gossip re-announce delivers the offer to late joiners for free
  and a PeerUpdated fires on start/stop — no separate gossip message needed.
- core: Start/Stop/ViewShare commands; host + viewer children tracked in the
  session, killed on stop/leave (kill_on_drop backstop). Viewer limit left to
  pixelpass's bandwidth-measured cap.
- UI: Share/Stop button (graceful "needs pixelpass" disabled state), Live badge
  + Watch on peer cards, Sharing badge on the self card. Verified by screenshot.
- config: optional `pixelpass_path` override (hand-editable).

Tests-green; the 2-machine gossip/remote path is not yet field-verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/messages.rs

@@ -31,6 +31,18 @@ pub enum CoreCommand {
     SetRecording(bool),
     /// Broadcast a room text-chat message. No-op when not in a call.
     SendChat(String),
+    /// Set the pixelpass binary location (config override, empty = use `$PATH`).
+    /// Sent at startup so screen-share can resolve the binary.
+    SetPixelpassPath(Option<String>),
+    /// Start sharing our screen: spawn a pixelpass host and announce its ticket
+    /// on our presence so the room can watch. No-op when not in a call.
+    StartScreenShare,
+    /// Stop sharing our screen: kill the pixelpass host and clear the presence
+    /// ticket. No-op when not sharing.
+    StopScreenShare,
+    /// Watch a peer's screen share: spawn a pixelpass viewer for `ticket` and
+    /// open it in a local player.
+    ViewShare(String),
 }
 
 #[derive(Debug, Clone)]
@@ -56,5 +68,9 @@ pub enum UiEvent {
     /// A room text-chat message arrived from a peer (never our own — local
     /// messages are echoed by the UI on send).
     ChatMessage { name: String, text: String },
+    /// Our own screen share started; the UI flips the Share button to "Stop".
+    ScreenShareStarted,
+    /// Our own screen share stopped (or failed to start).
+    ScreenShareStopped,
     Error(String),
 }